Integrity pdf

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South African Journal of Geomatics, Vol. Important concepts in quality control such as data availability, latency, multipath, atmospheric effects, and interference are discussed. These quality indicators are examined in the context of their capacity to indicate potential problems that can degrade the quality of real-time network positioning.

The issue of intelligent alerting is raised and an alternative strategy, based on the use of relative thresholds, is proposed with the aim of reducing the number of unnecessary alerts provided to operators. Africa is no exception to this global trend and several CORS networks have been or are in the process of being established Hedling et al. Quality control and integrity monitoring are important aspects of operating a real-time CORS network.

As such CORS operators have a responsibility to ensure that the raw data from their reference stations is of high quality, and can consistently satisfy the requirements of users. Quality breaches need to be detected and dealt with in a timely fashion including notifying users of the quality breach.

In some cases detecting problems is trivial. For example if a CORS is offline due to a receiver hardware or communications malfunction, the operator and users receive immediate feedback and can take appropriate action. However in cases of increased data latency, localised multipath, or irregular ionospheric activity the consequences of the quality breach may not be so obvious.

The data will be available but the quality will be degraded, resulting in users experiencing problems in the field. This paper reviews some important concepts in quality control that need to be considered when operating a real-time CORS network.

An alternative strategy to quality control using relative quality thresholds is introduced to support the concept of intelligent alerting.

The quality of the data received by the control centre needs to be continuously monitored to ensure that the data and information supplied to field users is reliable and fit-for-purpose. Issues that can have a negative effect on data quality include availability, cycle slips, latency, multipath, atmospheric effects and interference. These are examined in detail below. Implementation of an indicator is simple, the data is either available or it is not. A single epoch of missing data is not cause for concern, but if several consecutive epochs are missing an alert should be issued.

If the problem persists, the station may need to be removed from the network computation process, and the cause of the problem investigated. An alert should be sent to all users advising them that a particular station is offline. Common causes of missing data include receiver malfunction hardware or antenna related and problems with communication links. Alternatively, significant data gaps may be an indication of strong radio interference or significant atmospheric activity.

Indicators of data availability provide immediate feedback on a critical aspect of a CORS network and the indicators need to be monitored in real-time to inform users as problems occur as well as on a daily basis to detect patterns of behaviour. Cycle slips can be caused by physical obstructions to the satellite signal, a low Signal-to-Noise Ratio SNR , ionospheric scintillation, multipath, radio interference, or a malfunction of the receiver tracking loops Hofmann-Wellenhof et al.

High precision GNSS positioning requires that the integer number of carrier phase cycles must be known and this implies that cycle slips need to be detected and repaired.

Numerous algorithms for the detection and repair of cycle slips have been proposed in literature Blewitt, ; Kim and Langley, ; de Jong, The algorithm proposed by Blewitt uses the P code pseudoranges to automatically detect and correct for cycle slips in the wide-lane phase combination.

In post-processed analysis the current value is compared to the data both sides of the epoch to detect differences. In real-time analysis only past data can be used. Cycle slips at CORS sites need to be monitored as they are an early and robust indicator of data quality issues such as multipath, interference, or abnormal ionospheric conditions.

Multipath is an important quality issue for CORS sites. Multipath mitigation is normally done through careful site selection. While this is generally achievable, occasionally restrictions limit site selection, for example due to the need to have access to power and communications. In many cases, CORS antennas are placed on or near the roof of a building where the roof itself can be a significant source of multipath. To mitigate multipath from the antenna installation, the antenna needs to be elevated above the roof as much as possible, and receiver with multipath mitigation technology coupled with a choke ring antenna should also be used.

In any case, it is important to monitor the multipath levels at the CORS sites. The most common multipath indicators are the mp1 and mp2 linear combinations that define the pseudorange multipath on the L1 and L2 frequency Estey and Meertens, These indicators are measured in metres and have a standard threshold of 0. Values higher than 0. Data is transmitted from CORS sites to the control centre and consequently disseminated to users. CORS sites need to have a robust internet connection and enough bandwidth to handle data transfer during periods of peak load.

In an NRTK solution normally reference stations are used to compute the correction for the rover. If just one of the stations is experiencing unacceptable levels of latency it will have a negative effect on the whole NRTK computation. Ideally data latency needs to be under 1 second with a maximum of 2 seconds. Yan et al. The troposphere is a non-dispersive medium at radio frequencies and as such affects all GNSS signals equally.

In NRTK positioning algorithms, tropospheric biases are often lumped together with residual orbit biases to form a geometric bias term Seeber, On the other hand, the ionosphere is a dispersive medium and as such its influence depends on the frequency of the signal. The number of free electrons directly impacts on how much the signals are affected. The electron density in the ionosphere is measured by the Total Electron Content TEC , which is the number of electrons per square metre measured either vertically VTEC or along the signal path.

During periods of high solar activity, there is an increased level of TEC variability which can disturb the GNSS signals causing ionospheric scintillation. Figure 1 depicts the global VTEC map centred on Africa taken on 11th April solar maximum , and the same date in solar minimum.

Figure 1. Global VTEC map during solar maximum left and solar minimum right. It can be seen that around midday during the solar maximum the TEC is at extreme levels. Significant ionospheric events such as geo-magnetic storms can be predicted at least 24 hours in advance SWPC, , and for CORS operators it is important to be aware of these events. It must be noted that NRTK has been in wide commercial use for only the last five years, which was a period of solar minimum.

During that time ionospheric activity has been low, resulting in minimal ionospheric impact and reliable positioning performance. Hence it is critical to monitor NRTK performance over the next years leading up to and during the period of expected high ionospheric activity. One measure of ionospheric activity is the I95 index developed by Wanninger The I95 index is a statistical figure describing the amount of differential ionospheric biases as experienced by GNSS users.

Positioning algorithms applied to NRTK use sophisticated ionospheric models to account for the linear component of the ionosphere, however non-linear biases still remain. The I95L index attempts to describe the non-linear biases that remain between the network reference stations. Unintentional interference comes from radio signals transmitted at frequencies within or near the GNSS signal spectrum and can come from any radio frequency RF transmitter. Unintentional interference, much like multipath, can usually be avoided through careful site selection.

Intentional interference or jamming is a deliberate act of transmitting in the GNSS band. Jamming poses a serious threat to any GNSS receiver. Commercial jammers, although illegal in many countries, can be purchased cheaply over the internet. The problem with interference is that it is hard to detect.

If the source of the interference is strong enough, it can overpower the GNSS signals completely. One interference detection method was proposed by Weston et al. Log in with Facebook Log in with Google.

Remember me on this computer. Enter the email address you signed up with and we'll email you a reset link. Need an account? Click here to sign up. Download Free PDF. Eldar Rubinov. A short summary of this paper. South African Journal of Geomatics, Vol. Important concepts in quality control such as data availability, latency, multipath, atmospheric effects, and interference are discussed.

These quality indicators are examined in the context of their capacity to indicate potential problems that can degrade the quality of real-time network positioning. The issue of intelligent alerting is raised and an alternative strategy, based on the use of relative thresholds, is proposed with the aim of reducing the number of unnecessary alerts provided to operators.

Africa is no exception to this global trend and several CORS networks have been or are in the process of being established Hedling et al. Quality control and integrity monitoring are important aspects of operating a real-time CORS network. As such CORS operators have a responsibility to ensure that the raw data from their reference stations is of high quality, and can consistently satisfy the requirements of users.

Quality breaches need to be detected and dealt with in a timely fashion including notifying users of the quality breach. In some cases detecting problems is trivial. For example if a CORS is offline due to a receiver hardware or communications malfunction, the operator and users receive immediate feedback and can take appropriate action. However in cases of increased data latency, localised multipath, or irregular ionospheric activity the consequences of the quality breach may not be so obvious.

The data will be available but the quality will be degraded, resulting in users experiencing problems in the field. This paper reviews some important concepts in quality control that need to be considered when operating a real-time CORS network. An alternative strategy to quality control using relative quality thresholds is introduced to support the concept of intelligent alerting. The quality of the data received by the control centre needs to be continuously monitored to ensure that the data and information supplied to field users is reliable and fit-for-purpose.

Issues that can have a negative effect on data quality include availability, cycle slips, latency, multipath, atmospheric effects and interference. These are examined in detail below. Implementation of an indicator is simple, the data is either available or it is not. A single epoch of missing data is not cause for concern, but if several consecutive epochs are missing an alert should be issued. If the problem persists, the station may need to be removed from the network computation process, and the cause of the problem investigated.

An alert should be sent to all users advising them that a particular station is offline. Common causes of missing data include receiver malfunction hardware or antenna related and problems with communication links.

Alternatively, significant data gaps may be an indication of strong radio interference or significant atmospheric activity. Indicators of data availability provide immediate feedback on a critical aspect of a CORS network and the indicators need to be monitored in real-time to inform users as problems occur as well as on a daily basis to detect patterns of behaviour. Cycle slips can be caused by physical obstructions to the satellite signal, a low Signal-to-Noise Ratio SNR , ionospheric scintillation, multipath, radio interference, or a malfunction of the receiver tracking loops Hofmann-Wellenhof et al.

High precision GNSS positioning requires that the integer number of carrier phase cycles must be known and this implies that cycle slips need to be detected and repaired. Numerous algorithms for the detection and repair of cycle slips have been proposed in literature Blewitt, ; Kim and Langley, ; de Jong, The algorithm proposed by Blewitt uses the P code pseudoranges to automatically detect and correct for cycle slips in the wide-lane phase combination.

In post-processed analysis the current value is compared to the data both sides of the epoch to detect differences. In real-time analysis only past data can be used.

Cycle slips at CORS sites need to be monitored as they are an early and robust indicator of data quality issues such as multipath, interference, or abnormal ionospheric conditions. Multipath is an important quality issue for CORS sites.

Multipath mitigation is normally done through careful site selection. While this is generally achievable, occasionally restrictions limit site selection, for example due to the need to have access to power and communications. In many cases, CORS antennas are placed on or near the roof of a building where the roof itself can be a significant source of multipath.

To mitigate multipath from the antenna installation, the antenna needs to be elevated above the roof as much as possible, and receiver with multipath mitigation technology coupled with a choke ring antenna should also be used. In any case, it is important to monitor the multipath levels at the CORS sites. The most common multipath indicators are the mp1 and mp2 linear combinations that define the pseudorange multipath on the L1 and L2 frequency Estey and Meertens, These indicators are measured in metres and have a standard threshold of 0.

Values higher than 0. Data is transmitted from CORS sites to the control centre and consequently disseminated to users. CORS sites need to have a robust internet connection and enough bandwidth to handle data transfer during periods of peak load. In an NRTK solution normally reference stations are used to compute the correction for the rover. If just one of the stations is experiencing unacceptable levels of latency it will have a negative effect on the whole NRTK computation.

Ideally data latency needs to be under 1 second with a maximum of 2 seconds. Yan et al. The troposphere is a non-dispersive medium at radio frequencies and as such affects all GNSS signals equally. In NRTK positioning algorithms, tropospheric biases are often lumped together with residual orbit biases to form a geometric bias term Seeber, On the other hand, the ionosphere is a dispersive medium and as such its influence depends on the frequency of the signal. The number of free electrons directly impacts on how much the signals are affected.

The electron density in the ionosphere is measured by the Total Electron Content TEC , which is the number of electrons per square metre measured either vertically VTEC or along the signal path.

During periods of high solar activity, there is an increased level of TEC variability which can disturb the GNSS signals causing ionospheric scintillation. Figure 1 depicts the global VTEC map centred on Africa taken on 11th April solar maximum , and the same date in solar minimum.

Figure 1. Global VTEC map during solar maximum left and solar minimum right. It can be seen that around midday during the solar maximum the TEC is at extreme levels.

Significant ionospheric events such as geo-magnetic storms can be predicted at least 24 hours in advance SWPC, , and for CORS operators it is important to be aware of these events. It must be noted that NRTK has been in wide commercial use for only the last five years, which was a period of solar minimum.

During that time ionospheric activity has been low, resulting in minimal ionospheric impact and reliable positioning performance.

Hence it is critical to monitor NRTK performance over the next years leading up to and during the period of expected high ionospheric activity. One measure of ionospheric activity is the I95 index developed by Wanninger The I95 index is a statistical figure describing the amount of differential ionospheric biases as experienced by GNSS users.

Positioning algorithms applied to NRTK use sophisticated ionospheric models to account for the linear component of the ionosphere, however non-linear biases still remain. The I95L index attempts to describe the non-linear biases that remain between the network reference stations. Unintentional interference comes from radio signals transmitted at frequencies within or near the GNSS signal spectrum and can come from any radio frequency RF transmitter.

Unintentional interference, much like multipath, can usually be avoided through careful site selection. Intentional interference or jamming is a deliberate act of transmitting in the GNSS band. Jamming poses a serious threat to any GNSS receiver. Commercial jammers, although illegal in many countries, can be purchased cheaply over the internet.

The problem with interference is that it is hard to detect. If the source of the interference is strong enough, it can overpower the GNSS signals completely. One interference detection method was proposed by Weston et al. The procedure involves processing hourly RINEX files from the CORS sites and comparing the results against the published coordinates to see whether a significant change in position is observed.

Whilst effective, the method does not take into account that there are other factors that can influence the coordinates. This type of analysis is important as it can reveal the stability of the network, station velocities due to tectonic movement, and any datum inconsistencies due to local distortions.